This invention pertains broadly to the field of the semiconductor arts, and more particularly to the field of photonic porous silicon.
The band gap structure for single crystal silicon exhibits a conduction band minimum which does not have the same crystal momentum as the valence band maximum, yielding an indirect bad gap. Therefore, in silicon, radiative recombination can only take place with the assistance of a photon, making such transitions inefficient. This has prevented silicon from being used as a solid state source of light, unlike group III-V semiconductors which have a direct gap at the center of the Brillouin zone. A review of these materials properties can be found in S. M. Sze, Physics of Semiconductor Devices, 2nd. Edition (New York: John Wiley and Sons, 1981).
The discovery of photoluminescence in porous silicon has therefore generated a new optoelectronic material for study. A selected review of the fabrication techniques and properties of porous silicon can be found in the articles titled: xe2x80x9cSilicon quantum wire array fabrication by electrochemical and chemical dissolution of wafersxe2x80x9d by L. T. Canham, Appl. Phys. Lett., 57, 1046 (1990); xe2x80x9cVisible light emission due to quantum size effects in highly porous crystalline siliconxe2x80x9d by A. G. Cullis et al., Nature, 353, 335 (1991); xe2x80x9cVisible luminescence from silicon wafers subjected to stain etchesxe2x80x9d by R. W. Fathauer et al., Appl. Phys. Lett., 60, 995 (1992); xe2x80x9cDemonstration of photoluminescence in nonanodized siliconxe2x80x9d by J. Sarathy et al., Appl. Phys. Lett., 60, 1532 (1992): and xe2x80x9cPhotoluminescent thin-film porous silicon on sapphirexe2x80x9d, by W. B. Dubbelday et al., Appl. Phys. Lett., 62, 1694 (1993).
Porous silicon can be formed, for example, by using electrochemical etching, photochemical etching or stain etching of either pure silicon substrates or of the silicon of silicon on transparent substrates (e.g. silicon-on-sapphire or silicon-on-quartz) as described in the above references and the cited co-pending patent application Ser. No. 08/118,900 of Russell et al. Such techniques can produce porous silicon typically containing mechanically fragile structures on the order of approximately 5 nm or less in size. Other materials such as germanium, silicon-germanium alloys and the like may also be etched into porous form. The utilized materials may be suitably patterned lithographically prior to the etch to define device structures or to confine the region desired to be exposed to the etch solution.
In FIG. 1, a scanning electron micrograph of electrochemically prepared porous silicon is shown. This porous silicon was prepared, as is commonly practiced in the art, by electrochemical dissolution of silicon in a solution of 48% hydrofluoric acid and 95% ethyl alcohol in a ratio of about 1:1 with a current flow in the range of about 0.1 to 10 mA/cm2. FIG. 1 depicts the typical resulting structure of this process, showing dendritic-like silicon structures (lighter regions) surrounded by a large density of voids (darker regions), i.e. a porosity of greater than about 75%. The silicon structures shown in FIG. 1 also have similar structures on a smaller length scale, dimensions of approximately less than 10 nm, not observable by the scanning electron micrograph technique.
The photonic (light-emitting) properties of the porous silicon have been attributed to these smaller structures. Further details on the formation of these silicon structures and their light emitting properties are contained in the cited co-pending U.S. patent application by S. D. Russell et al., titled xe2x80x9cPhotonic Silicon on a Transparent Substratexe2x80x9d United States Patent and Trademark Office Ser. No. 08/118,900 incorporated herein by reference.
The typical emission spectrum of porous silicon is in the red, orange and yellow region, i.e. 500 to 750 nm, although green and blue emissions have also been demonstrated. Blue shift of the peak emission wavelength has been shown by increased oxidation and etching of the porous silicon as described in xe2x80x9cControl of porous Si photoluminescence through dry oxidationxe2x80x9d by S. Shih et al., Appl. Phys. Lett., 60, 833 (1992) and in xe2x80x9cLarge blue shift of light emitting porous silicon by boiling water treatmentxe2x80x9d by X. Y. Hou et al., Appl. Phys. Lett., 62, 1097 (1993).
The article titled xe2x80x9cReversible Luminescence Quenching of Porous Si by Solventsxe2x80x9d by J. M. Lauerhaas et al., J. Am. Chem. Soc., 114, 1911 (1992) discloses that a reversible quenching of the photoluminescence is obtained from porous silicon fabricated in bulk silicon due to surface adsorbates. The degree of quenching nominally scales with the solvent dipole moment. Furthermore, it has been discovered that, in many cases, the quenching of the light emitting property is not reversible when porous silicon is contacted by solutions and chemical elements commonly used in semiconductor processing. These effects demonstrate that the light emission of porous silicon is chemically fragile, i.e. susceptible to being changed by a chemical element or compound. In addition, it is known that porous silicon can be thermally fragile, as the heating of the porous silicon structures and/or devices to temperatures approaching about 300xc2x0 C. and above permanently destroys the light emitting (photonic) properties of the porous silicon.
At this time the light emitting mechanism is not fully understood. The scientific controversy surrounding the detailed physical mechanism behind the light emission has not, however, hindered the ability to fabricate porous silicon layers and useful light emitting devices using this technology as described in xe2x80x9cVisible electroluminescence from porous siliconxe2x80x9d by N. Koshida et al., Appl. Phys. Lett., 60, 347 (1992); xe2x80x9cNew Results on Electroluminescence from Porous Siliconxe2x80x9d by P. Steiner et al., in Microcrystalline Semiconductors: Materials Science and Devices, Materials Research Society Proceedings, 283, 343 (1993) and in xe2x80x9cCurrent injection mechanism for porous-silicon transparent surface light-emitting diodesxe2x80x9d by H. P. Maruska et al., Appl. Phys. Lett. 61, 1338 (1992).
The abstract titled xe2x80x9cProgress in the Development of Porous Silicon Light Emittersxe2x80x9d by P. M. Fauchet et al., 1995 Electronic Materials Conference, Technical Program with Abstracts, page A51, Jun. 21, 1995, notes, that the efficiency for porous silicon light-emitting diodes remains low xe2x80x9cdue to the difficulty in making solid state contacts to a highly porous structurexe2x80x9d.
It is known to use evaporated or sputter-deposited layers of semi-transparent gold or indium tin oxide (ITO) to make electrical contact to porous silicon layers and device structures. These techniques are line-of-sight deposition techniques that do not fill the irregular matrix of voids inherent of the porous silicon, due, in part, to the large particle size and the directionality of deposited material.
FIG. 2 is a cross-section of a silicon layer 10 in which is formed a porous silicon region 12 consisting of voids 14 and silicon structures 16. According to a prior art technique, porous silicon region 12 is covered by an electron beam sputtered conducting layer 18 of a conductive metal such as indium-tin-oxide (typically 95% indium oxide, 5% tin oxide). As can be seen, conductive metal 18 does not fill voids 14 of porous silicon region 12, preventing efficient electrical contact between conducting metal 18 and silicon structures 16.
In U.S. Pat. No. 5,331,180, titled xe2x80x9cPorous Semiconductor Light Emitting Devicexe2x80x9d, M. Yamada et al. teaches the use of a conductive polymer layer as a means to make electrical contact to porous silicon or porous silicon-carbide structures and to mechanically support the fragile porous silicon. In Yamada et al""s embodiment, the conductive polymer layer binds to the top surface of the porous silicon as well as the xe2x80x9cupper regionsxe2x80x9d of the pores or voids of the porous silicon, however the polymer conductor is not flowed to substantially fill these voids.
While prior art techniques of making electrical contact to porous silicon are known to exist, improving the efficiency of these contacts is a desired goal.
Alternative techniques are therefore desired to make electrical contact to the mechanically, chemically and thermally fragile porous structures in such a manner as to preserve and enhance their photonic properties.
The invention is a method of making improved electrical contact to porous photonic materials, such as porous silicon, using conductive materials that interpenetrate the porous materials without damaging their photonic properties. This intercalation process may use gaseous, liquid or solid components to form extensive conductive contacts to the structures of the porous materials to thereby enhance or promote their photonic characteristics. The improved electrical interconnection between the mechanically, chemically and thermally fragile porous materials and device electrodes will allow an increase in the intensity of the light emitted by the photonic structures of the porous materials.
An object of this invention is to improve electrical contact to porous photonic materials.
Another object of this invention is to improve electrical contact to porous silicon.
Another object of this invention is to make electrical contact to porous silicon using gaseous, liquid or solid components.
Another object of this invention is to make electrical contact to porous silicon without mechanically damaging the light-emitting (photonic) properties of the porous silicon.
Another object of this invention is to make electrical contact to porous silicon without chemically damaging the light emitting (photonic) properties of the porous silicon.
Yet another object of this invention is to make electrical contact to porous silicon without thermally damaging the light emitting (photonic) properties of the porous silicon.
Another object of this invention is to make electrical contact to porous silicon using intercalated conductive materials.
Yet another object of this invention is to make electrical contact to porous silicon using conductive materials that interpenetrate the silicon structures of porous silicon to improve electrical contact to these structures.
These and other objects of the invention will become more readily apparent from the following specification and claims when taken in conjunction with the appended drawings.